From EP 0 255 268 A2 there is known a method for operating a continuous coating installation for the coating of workpieces which, in a cataphoretic bath, are fed continuously in the feed direction and kept apart. The bath has a dipping region of sufficient size for the complete dipping of a plurality of workpieces which are spaced apart. In order to avoid voltage flashovers with spark formation and defects on the electrophoretically produced layers, for example holes or unevenness, a DC voltage is increased linearly to a coating voltage in an infeed section for the duration of a short run-up time. In the following sections of the dipping region, in which the actual coating takes place, in a first embodiment the voltage is then kept constant in each case at the value of the coating voltage or, in a second embodiment, the voltage is increased stepwise. However, the coating on the workpieces acts as an insulating layer on their surface. The thickness of the insulating layer increases with the coating time. When a constant DC voltage is applied (first embodiment) in the second section and, where appropriate, in further sections of the dipping region, the coating rate is dependent on the conductivity of the workpiece surface and the current density is accordingly initially very high. Owing to the increasing thickness of the insulating layer, it decreases approximately exponentially with the coating time until saturation occurs or the electric circuit is broken. The increasing insulating layer thickness therefore leads to a marked lengthening of the coating period as a whole. Therefore, correspondingly long dipping regions are required in order to lengthen the residence time of the workpieces. The high current peaks at the beginning of the coating operation require the use of large, and thus expensive, rectifiers. The constant DC voltage during the actual coating time in the sections of the dipping region provided therefor also leads to different layer thicknesses in the case of large workpiece surfaces than in the case of small workpiece surfaces. In addition, increasing the voltage only during the short run-up time in the first section of the dipping region impairs the quality of the coating on the surface of the workpieces. The stepwise increase in the DC voltage (second embodiment) from the second section of the dipping region leads to current jumps. The optimum intensity of current for keeping the current density constant is therefore not always applied to the workpieces.
In order to adjust the voltage for the coating, the method of current density stabilisation is known from other continuous coating installation known on the market. In this method, the voltage is adjusted in dependence on the dipped surface of the workpiece. However, adjustment of the coating rate is not possible thereby.
In order to coat cavities, in particular closed tubular parts, it is also known to apply short voltage pulses with a high voltage between the workpiece and the electrode in order to permit a wrap-around, in particular an internal coating, even with cavity depths greater than 500 mm. The voltages are limited to 450 V in the case of lacquer coatings because the lacquer can coagulate at higher voltages. This method is not suitable for compensating for the decrease in the electrical conductivity of the workpiece surface as a result of the increase in the insulating layer thickness.
In cyclic installations known from the market for the coating of workpieces, the workpieces are dipped cyclically in a region of the bath and maintained therein. For the duration of the dipping, a substantially constant voltage is applied, by means of a voltage source, between the dipped workpiece and at least one electrode in the bath. In order to counteract the problem of the decrease in the coating rate with the coating time because of the increasing thickness of the insulating layer, longer cycle times are provided for the coating, as a result of which the coating operation as a whole is markedly lengthened.
The invention is directed to resolving these and other matters.
An object of the present invention is to provide a method and a coating installation of the type mentioned at the beginning with which workpieces can be provided as simply as possible with a high-quality coating, in particular having a specifiable layer density and a specifiable layer thickness.
In the case of the method according to the invention, that object is achieved by increasing the DC voltage continuously, in a substantially stepless manner, for virtually the entire coating period so that the coating current density at the workpiece surface remains substantially constant over time.
According to the invention, therefore, a reduction in the conductivity of the workpiece surface as a result of the increase in the thickness of the coating is counteracted for virtually the entire coating period by continuously increasing the voltage so that the current and the flux of the media particles, particles here being understood as being both suspended and dispersed particles, and accordingly the coating rate, remain virtually constant over the coating period. As a result, a controlled, homogeneous application of the media particles to the workpiece surface, preferably with a specified density and layer thickness, is achieved for virtually the entire coating time. Because the layer thickness, in dependence on the coating medium, is proportional to the supplied electric charge, it can readily be determined. Moreover, as a result of the controlled continuous voltage increase there are no current peaks, so that the voltage source and any contacts, in particular sliding contacts when a continuous installation is used, are subjected to less stress and smaller rectifiers can be used. In continuous installations in particular, the risk of voltage flashovers as a result of spark formation is thus also reduced. The resulting current profile, which is virtually constant over time, additionally leads to a reduction in harmonics when AC voltage is used to supply the voltage source. Moreover, a markedly better effective power factor can be achieved because no-load times of the voltage source are reduced as a result of the virtually constant current profile. When used in conjunction with continuous installations, the dipping regions can be made shorter in order to achieve the same layer thicknesses as in the continuous installations known from the prior art with shorter coating periods. In a corresponding manner, the cycle times can be correspondingly shorter when cyclic installations are used.